There are known many analytical methods in which specific components are determined by detecting hydrogen peroxide which is formed by the reaction of said specific components. Among them, there is a method using leuco dyes having an imidazole nucleus.
One mol of a dye is formed from one mol of hydrogen peroxide interacting with one mol of a leuco dye and the dye has a high molecular extinction coefficient so that they have the advantage in that their detection sensitivity is high. On the other hand, they have a disadvantage in that it is difficult to make an accurate measurement since dyes formed by the leuco dyes fade rapidly after color formation.
It was found that the fading of the dye produced by the leuco dye can be substantially prevented by dissolving the leuco dye in a hydrophobic solvent, dispersing the solution in a hydrophilic medium and incorporating the dispersion in a water-permeable layer (see, JP-A-63-247657 corresponding to EP 285095A2) (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, when a leuco dye is dissolved in a hydrophobic solvent and then dispersed in a hydrophilic medium, the color forming rate is reduced.
Further, the mean particle size of the dispersion is increased, when a dispersion of a leuco dye dissolved in a hydrophobic solvent is stored at 40.degree. C. for two or three hours or at 5.degree. C. for about one week even if a conventional anionic surfactant is used. Namely, the dispersion is unstable.